This invention relates to time division multiple access (TDMA) communications systems, and in particular, to the dynamic recovery of a TDMA signal during an undetected marker sequence.
In a TDMA communications system, a number of transmitters can transmit on the same frequency channel, but at different times. A remote receiver for receiving a particular transmitted signal knows before hand at what approximate time the transmitted signal will occur and is enabled only during that time. Details for such a TDMA communications system are provided in U.S. Pat. No. 5,598,419, which is incorporated by reference herein.
The TDMA technique makes very efficient use of the frequency spectrum since multiple users may use the same radio-frequency (RF) channel at the same time without interfering with one another. FIG. 1 illustrates one type of TDMA system where a plurality of remote portable stations 10, 11, 12, and 13 share a same frequency channel while transmitting to and receiving from a base station 14. The portable stations 10-13 can be, for example, wireless telephones, and the base station 14 can be a high-power transponder base cell. All communications between portable stations 10-13 are routed through base station 14. For communication, each of portable stations 10-13 and base station 14 contains a transceiver which operates in various modes, such as set-up/control mode, transmit/receive mode, data mode, etc.
One embodiment for such a TDMA system is the Personal Handy Phone System (PHS), the requirements for which are described in RCR Standard-28, incorporated herein by reference. As an alternate embodiment, such a TDMA system may be the ETSI DECT standard, also incorporated herein by reference. Further, slow frequency hopping systems, compliant with C.F.R. Title 47, part 15, and intended for the U.S. ISM-bands, may be derived from the aforementioned formal standards.
In a TDMA system, each remote transceiver (e.g., portable stations 10-13), when active, is allocated certain times slots within which it may transmit a bursted signal or receive a bursted signal. FIG. 2 illustrates a frame 16 containing slots 0 through 7, where frame 16 is repeated on a signal frequency channel. The period of frame 16 may be, for example, 5 msecs. Assuming all four portable stations 10-13 in FIG. 1 are being actively used at the same time, portable stations 10, 11, 12, and 13 may be allocated slots 0, 1, 2, and 3, respectively, for transmitting bursted signals to base station 14, while portable stations 10, 11, 12, and 13 may be allocated slots 4, 5, 6, and 7, respectively, for receiving bursted signals from base station 14. The amount of information stored in each of portable stations 10-13 during a frame period is transmitted in a burst within a single slot. In one embodiment, the bit rate of the transmitted bits in a slot is approximately 384K bits per second, and the corresponding symbol rate is, therefore, 192K symbols per second.
A sample protocol 18 which dictates the information required to be transmitted during a single slot is also shown in FIG. 2. Protocol 18 may consist of a ramp-up (R) field 20, a start symbol (SS) field 21, a preamble and/or clock recovery field 22, a unique word or slot synchronization field 23, a data field 24 (typically used as a traffic channel or TCH), a CRC (for error correction and verification) field 25, and a guard bits field 26. The lengths and types of fields in a protocol vary depending on the mode of the transceiver (e.g., set-up/control mode, transmit/receive mode, etc.). While in the traffic mode, where voice is to be transmitted, data field 24 contains audio or voice data.
Modern, digital TDMA communication systems require very accurate synchronization in the time domain. To achieve this, such systems commonly employ a known marker sequence within the TDMA burst architecture. In the case of the conventional TDMA system (such as the PHS) described above, the xe2x80x9cunique wordxe2x80x9d of protocol 18 is used as the marker sequence for each burst. The unique word is chosen to have special orthogonal properties which yield a sharp peak during an auto-correlation process performed at a transceiver. When the marker sequence is detected by the transceiver, the transceiver""s time-base is re-aligned to the incoming signal. Thus, the transceiver completely re-synchronizes on each frame. Afterwards, the payload data in the burst can be properly recovered. If environmental conditions (such as noise or fading) exist which adversely affect the RF channel, however, proper detection of the marker sequence may not be possible even though the payload data in the burst may be unaffected.
In previously developed TDMA systems, the payload data in the burst cannot be decoded if the marker sequence is not detected. These previously developed systems treat data for voice and non-voice (e.g., control) communication in the same way when the marker sequence is undetected for a burstxe2x80x94i.e., the payload data for that burst is not processed. For non-voice communication, this is not problematic because the control data can be re-transmitted. For voice communication, however, the payload data of a burst is lost forever. This can cause a significant degradation in sound quality for voice communication, which in turn, may lead to the dissatisfaction, and ultimately, the loss of customers for a provider operating such a previously developed TDMA system.
What is needed is a system and method by which the payload data of a TDMA signal can be recovered even when the marker sequence is undetected.
A TDMA transceiver architecture is described which dynamically recovers the payload data of a TDMA signal even when the signal""s marker sequence is not detected. This transceiver architecture is based on a high-inertia TDMA system. That is, once synchronization is achieved between a portable station and a base station, the portable station does not completely re-synchronize on each frame, but instead operates on its own internal crystal (clock) and adjusts its time-base as appropriate during each frame to stay in synchronization with the base station. Accordingly, the portable station remains well-aligned on a frame-by-frame basis. Thus, if the unique word is undetected (for example, due to undesirable RF channel conditions), the portable station can nonetheless determine the location of the payload data in the signal due to that station""s independent, free-running time-base.
In one embodiment, if the unique word for a slot is undetected, a TDMA transceiver of the present invention assumes that voice data is carried in the payload of such slot. The TDMA transceiver then defines the channel identifier (CI) field for the slot as one which identifies the payload as voice data. The TDMA transceiver processes the payload accordingly, whether or not the payload is actually voice data. If indeed the payload carries voice data, such data is recovered and not lost, and therefore, sound quality is not degraded when the marker sequence is not detected.
Furthermore, the TDMA transceiver includes a mechanism for detecting errors. Specifically, such TDMA transceiver uses the CRC (for error detection and verification) field as a check on the data carried within the payload field. Thus, if it is assumed that the payload carries voice data, but the CRC field indicates otherwise, the processed payload is not output as audio information, but rather, will be discarded.
According to one embodiment of the present invention, a system includes a modem which receives a burst of a time division multiple access (TDMA) signal. A burst mode controller, connected to the modem, is operable to detect a unique word in the burst. If the unique word is not detected, the burst mode controller loads a predetermined bit pattern into a channel identifier field of the burst. The predetermined bit pattern indicates that a payload field of the burst contains voice data.
According to another embodiment of the present invention, a method includes the following steps: receiving a burst of a time division multiple access (TDMA) signal; attempting to detect a unique word in the burst; and loading a predetermined bit pattern into a channel identifier field of the burst if the unique word is not detected, the predetermined bit pattern indicating that a payload field of the burst contains voice data.
Thus, an important technical advantage of the present invention includes the ability to recover the payload data of a TDMA signal even when that signal""s marker sequence is undetected. Another important technical advantage of the present invention includes an error-detection mechanism. Accordingly, the present invention improves the quality of voice communication in a TDMA system, thereby promoting customer satisfaction and loyalty for a provider. Other important technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.